A purified bovine serum albumin preparation contains an insulin-like growth factor (IGF) binding protein-3 fragment that forms ternary complexes selectively with IGF-II and the acid-labile subunit

Among the six insulin-like growth factor binding proteins (IGFBP), only IGF BP-3 and IGFBP-5 form ternary complexes with IGFs and the acid-labile sunbu nit (ALS). In a commercial, highly-purified BSA preparation, ternary comple x formation was detected using radio-labeled IGF-II and human serum-derived ALS, with precipitation by ALS antiserum. In contrast, no complexes with r adio-labeled IGF-I were detected under the same conditions. Size-fractionat ion of the BSA on Superose-le showed the peak of ternary complex forming ac tivity at approximately 30 kDa. To purify the active factor, a solution of the BSA was pumped onto a [Gly(1)]IGF-II affinity column, and eluted fracti ons were lyophilized and applied to a C18 HPLC column. The eluted fractions showing ternary complex forming activity maintained a preference for IGF-I I in forming ternary complexes and a slight preference in forming binary co mplexes with IGF-II rather than IGF-I. By silver staining after non-reducin g SDS-PAGE, the peak activity in the HPLC-eluted fractions appeared as 30 k Da and 21-24 kDa bands. Amino-terminal sequencing of this peak activity rev ealed bovine IGFBP-3. These results demonstrate that amino-terminal proteol yzed bovine IGFBP-3 is present in a highly purified BSA preparation. In con trast to intact human IGFBP-3 and IGFBP-5, this form of bovine IGFBP-3 form s ternary complexes preferentially with IGF-II rather than IGF-I. (C) 2000 Harcourt Publishers Ltd.